linux-kernel - Re: + mm-compaction-avoid-premature-range-skip-in-isolate_migratepages

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Date:	Wed, 29 Oct 2014 15:04:02 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	cl@...ux.com, mgorman@...e.de, mina86@...a86.com,
	minchan@...nel.org, n-horiguchi@...jp.nec.com, riel@...hat.com,
	rientjes@...gle.com, zhangyanfei@...fujitsu.com,
	mm-commits@...r.kernel.org
Subject: Re: + mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
 added to -mm tree

On 10/24/2014 05:00 AM, Joonsoo Kim wrote:
> On Thu, Oct 23, 2014 at 10:39:45AM +0200, Vlastimil Babka wrote:
>> On 10/23/2014 10:15 AM, Joonsoo Kim wrote:
>>> On Tue, Oct 14, 2014 at 01:53:44PM -0700, akpm@...ux-foundation.org wrote:
>>>>
>>>> The patch titled
>>>>       Subject: mm/compaction.c: avoid premature range skip in isolate_migratepages_range
>>>> has been added to the -mm tree.  Its filename is
>>>>       mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
>>>>
>>>> This patch should soon appear at
>>>>      http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
>>>> and later at
>>>>      http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-avoid-premature-range-skip-in-isolate_migratepages_range.patch
>>>>
>>>> Before you just go and hit "reply", please:
>>>>     a) Consider who else should be cc'ed
>>>>     b) Prefer to cc a suitable mailing list as well
>>>>     c) Ideally: find the original patch on the mailing list and do a
>>>>        reply-to-all to that, adding suitable additional cc's
>>>>
>>>> *** Remember to use Documentation/SubmitChecklist when testing your code ***
>>>>
>>>> The -mm tree is included into linux-next and is updated
>>>> there every 3-4 working days
>>>>
>>>> ------------------------------------------------------
>>>> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>>>> Subject: mm/compaction.c: avoid premature range skip in isolate_migratepages_range
>>>>
>>>> commit edc2ca612496 ("mm, compaction: move pageblock checks up from
>>>> isolate_migratepages_range()") commonizes isolate_migratepages variants
>>>> and make them use isolate_migratepages_block().
>>>>
>>>> isolate_migratepages_block() could stop the execution when enough pages
>>>> are isolated, but, there is no code in isolate_migratepages_range() to
>>>> handle this case.  In the result, even if isolate_migratepages_block()
>>>> returns prematurely without checking all pages in the range,
>>>>
>>>> isolate_migratepages_block() is called repeately on the following
>>>> pageblock and some pages in the previous range are skipped to check.
>>>> Then, CMA is failed frequently due to this fact.
>>>>
>>>> To fix this problem, this patch let isolate_migratepages_range() know the
>>>> situation that enough pages are isolated and stop the isolation in that
>>>> case.
>>>>
>>>> Note that isolate_migratepages() has no such problem, because, it always
>>>> stops the isolation after just one call of isolate_migratepages_block().
>>>>
>>>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>>>> Cc: Vlastimil Babka <vbabka@...e.cz>
>>>> Cc: David Rientjes <rientjes@...gle.com>
>>>> Cc: Minchan Kim <minchan@...nel.org>
>>>> Cc: Michal Nazarewicz <mina86@...a86.com>
>>>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>>>> Cc: Christoph Lameter <cl@...ux.com>
>>>> Cc: Rik van Riel <riel@...hat.com>
>>>> Cc: Mel Gorman <mgorman@...e.de>
>>>> Cc: Zhang Yanfei <zhangyanfei@...fujitsu.com>
>>>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>>
>> Sorry for the trouble. But I think a more robust and future-proof fix
>> would be a check such as: if (pfn < block_end_pfn) break;
>> This should catch any reason where isolate_migratepages_block() did not
>> finish whole pageblock, and which was not fatal enough to return pfn==0.
>> However currently this seems to happen only due to isolating too much,
>> so your patch should work.
>> So it's up to you if you want to make the check more generic now, or
>> later after this bug is fixed for 3.18.
>
> 'if (pfn < block_end_pfn) break;' has one problem.
> If we have enough isolated pages and reach at block_end_pfn,
> we can't stop with above check.

Oh, right.

> More proper check may be as following.
> 'if (pfn < block_end_pfn ||
> cc->nr_migratepages == COMPACT_CLUSTER_MAX) break;'
> But, as you mentioned, there is no case where 'pfn < block_end_pfn'
> now, so I'd like to remain the patch as is.

Yep, that would only make things uglier :/ So I'm for merging your patch 
in 3.18 cycle to prevent breakage.

> Thanks.
>

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